The invention relates to a method for controlling a handover in a radio communications system.
A mobile radio communications system typically comprises a multiplicity of transceiver stations or base stations which exchange radio signals with mobile terminals which are located within the range of these stations, and at least one administration unit which switches user data between mobile terminals which are located within the range of different base stations, or between a mobile terminal and a fixed network.
A “W-CDMA” mobile radio communications system such as the TDD mode of the forthcoming third-generation UMTS mobile radio system (Universal Mobile Telecommunication System) provides the facility for a mobile terminal to communicate at a given time with a subset of these transceiver stations by receiving user data associated with a single call connection on different channels of a plurality of these stations, and, conversely, user data transmitted by the terminal are also received by this plurality of stations. This subset is also designated as the active set.
The purpose of this intrinsically redundant transmission is the avoidance of transmission gaps if the mobile terminal moves out of range of a station with which it is communicating; if it is communicating with one single station only, the radio link is interrupted when it moves outside its range and the user data transmission from and to the terminal cannot continue until a different station is allocated to said terminal, the user data which are to be transmitted to the terminal are switched to this station, and the terminal and the new station can be synchronized with one another. On the other hand, in the event of simultaneous communication with a plurality of stations, failure of the radio link between the mobile terminal and one of the stations does not yet result in interruption of the transmission, since said transmission continues to run unchanged via the other stations, while an alternative station is defined for the failed station and communication is set up with said alternative station. This type of station changeover with continuation of the communication with one or more other stations is referred to as “soft” handover.
However, simultaneous communication with a plurality of stations entails a substantial loading of the switching and transmission capacity of the radio communications system. If each active terminal occupies transmission channels of a plurality of stations, the number of mobile radio subscribers which can be served simultaneously with a given network infrastructure is of course considerably smaller than if each terminal uses only one channel of a station. Similarly, in the mobile terminal, the need to process receive signals from a plurality of channels can result in increased power consumption and therefore in a reduction of the network-independent usage time of a terminal of this type. A compromise between transmission reliability and transmission capacity therefore needs to be found, which is usually such that, for a radio communications system of this type, the maximum number of stations which may belong to the active set is defined as a small value of e.g. 2 or 3.
The stations which belong to the active set are identified using regularly repeated evaluations of the quality of transmission between the mobile terminal and the stations which are able to communicate with it. For this purpose, the mobile terminal measures the quality of radio signals which it receives from these stations. Stations which do not belong to the active set, but whose transmission quality, from one evaluation to the next, has become higher than that of a station of the active set, are reported to an administration unit which adds them to the active set and in return excludes the poorer station.
Simulation experiments have shown that, in a system of this type, it may often be the case that the quality of transmission between the mobile terminal and all stations of the active set becomes so poor between two transmission quality evaluations that the entire active set must be replaced at once. This results in a transmission gap. Such a situation may arise in particular if the mobile terminal is moving in an urban environment, where roads bordered by buildings can transport a radio signal over long distances in a longitudinal direction, but will screen it in a transverse direction, so that a terminal located on a road of this type may have an active set in which only relatively far-distant stations are located. As soon as the terminal moves into a transverse road, transmission from and to all the stations may be virtually simultaneously interrupted.
From WO99/04593, a method is known for the selection of base stations for communication with a mobile station, in which the mobile station receives signals from a plurality of base stations, referred to as the “candidate set”, identifies a relevant receive strength and compares it with a first threshold value. Base stations whose receive strengths lie above the threshold value are reported to a base station controller as suitable for inclusion in the active set of base stations. The mobile station identifies the need for changes to the current active set through measurements of the energies of pilot signals of the base stations of the active set and the candidate set, and through dynamic adaptation of the threshold values.
One potential object for the invention is to provide a method for controlling a soft handover with which high immunity of the transmission to interruptions is achieved without loading the switching and transmission capacity due to substantially increased redundancy, and without resulting in a significant increase in the power requirement of the terminal.